# Transmission Line Question - RF Cafe Forums

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mikerauh
Post subject: Transmission Line Question Posted: Fri Mar 07, 2008 2:46 pm

Lieutenant

Joined: Fri Mar 07, 2008 2:29 pm
Posts: 3
In his book, Prof. Guillermo Gonzalez writes:

V(x)=V+(x) + V-(x) and

I(x)=I+(x) - I-(x)

Why is there a minus sign in the second equation? The equations say the voltage at a point on the transmission line is the sum of the forward wave voltage V+(x) and the reflected wave voltage V-(x). But the current is the difference of the current of the forward wave I+(x) and the current of the reflected wave I-(x). It seems to me the voltage and the current are vector quantities, i.e. they have a phase. So why the minus sign? Why not just sum them?

Mike Rauh

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fred47
Post subject: Posted: Fri Mar 07, 2008 4:30 pm

General

Joined: Wed Feb 22, 2006 3:51 pm
Posts: 104
If I understand it correctly, it's to make the direction of the power flow work out correctly. Power doesn't really have a phase, but power flow definitely has a direction, and I think the signs are set up to give that.

Hope this helps!
Fred

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mikerauh
Post subject: Posted: Fri Mar 07, 2008 5:18 pm

Lieutenant

Joined: Fri Mar 07, 2008 2:29 pm
Posts: 3
Fred,

You are correct. The apparent current has to be defined as the incident wave current minus the reflected wave current to get the power relationships right.

Thank you.

Mike

Posted  11/12/2012

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